1. Technical Field
The present disclosure relates to a method of forming an optical waveguide on a circuit substrate.
2. Related Art
In various electronic circuits, with enhancement of a signal carrying speed (increase of a higher frequency), optical/electrical hybrid circuits a part of which is replaced with optical interconnection using the optical waveguide in place of the conductive wirings made of copper has been developed.
In FIGS. 1A and 1B, a typical example of an optical waveguide mounted on a circuit substrate is shown. As shown in FIG. 1A, both surfaces of a core layer 12 serving as a light traveling path of an optical waveguide 10 are covered with a lower cladding layer 14a and an upper cladding layer 14b, and the optical waveguide 10 has a light polarizing plate 16 on an inclined surface of the end portion. As shown in FIG. 1B, the optical waveguide 10 is mounted on a circuit substrate 20 by an adhesive 18. For example, the circuit substrate 20 is constructed by forming an insulating layer 26 and a wiring layer 28 as a build-up layer on both surfaces of a double-sided copper-clad core substrate up to a predetermined number of layers. According to the double-sided copper-clad core substrate, a copper foil 24 is pasted onto both surfaces of a core material 22. Then, a solder land 30 and a solder resist layer 32 used for the external connection are formed on the upper surface. The optical waveguide 10 is adhered onto the uppermost insulating layer 26 by the adhesive 18, for example.
As disclosed in JP-A-2000-199827, for example, in the known method of forming the optical waveguide, a triple-layered structure consisting of lower cladding layer/core layer/upper cladding layer is formed by three steps of laminating and curing a lower cladding sheet, laminating and patterning a core sheet, and laminating and curing an upper cladding sheet, and then this triple-layered structure is bonded to the circuit substrate with the adhesive.
According to the above method, much takt time and cost are required for the above three steps, and also the upper/lower cladding layers formed by laminating the cladding sheet have a certain thickness respectively. Therefore, the above method has such a disadvantage that this method is unsuitable for the slimming down of the circuit.
Meanwhile, as described in JP-A-2004-341454, an upper cladding layer and a core layer are sequentially laminated on a metal layer for wiring layer formation, then a cover film for protection is pasted on the core layer, and then a V-groove is formed on the core layer by applying the cutting process. The unnecessary core layer of the optical waveguide in one side of the V-groove is melted and removed while the core layer on the other side of the V-groove remains as the core layer of the optical waveguide, so that a laminated product is formed. This laminated product has a double-layered structure such that the upper cladding layer and the core layer are laminated on the metal layer for forming the wiring layer. Then, the core layer side the laminated product is adhered onto the circuit substrate via the adhesive. Accordingly, the optical waveguide consisting of upper cladding layer (laminated layer)/core layer (laminated layer)/lower cladding layer (adhesive layer) is formed on the circuit substrate. The electric circuit may be provided on the upper surface of the circuit substrate to which the laminated product is adhered.
However, the method described in JP-A-2004-341454 has the following disadvantages.
Namely, the substrate on which the upper cladding layer and the core layer are formed always requires the metal layer for forming the wiring layer, and thus the laminated product is restricted to such a structure that a predetermined wiring layer is provided directly on the upper cladding layer of the completed optical waveguide. Therefore, the laminated product must be designed integrally with the circuits on the circuit substrate. This leads to lack of versatility. Alternately, although not described in JP-A-2004-341454, even though the laminated product is used after the metal layer is removed, the wet process is required to melt/remove the metal layer and thus steps become complicated.
Also, a bottom portion of the V-groove on the core layer formed by the cutting process must match up with a boundary between the upper cladding layer and the core layer. Thus, processing accuracy is required in forming the V-groove.